I will make this very simple. Put very bluntly, scales are a set of notes that sound good together.
For instance, Do Re Mi... (or Sa Re Ga... if you prefer) is a scale (the major scale). These notes sound good together.
You can learn scales to use them to play guitar solos; it's not necessary to have to learn scales to play guitar solos but if you do learn them you may find it easier to play and compose solos because you will know what notes go well together and doing so you may be able to get a nicer sounding solo.
There are various types of scales and many versions of those too but there are 5 basic scales
2) Minor Pentatonic
4) Major Pentatonic
The Major scale as mentioned before is the most widely used scale in music (not just western...even traditional Indian music uses this scale). The most common scales used in rock/heavy metal/country music are the Minor Pentatonic and the Blues scale (which is also used in blues...duh!). The Major Pentatonic is a very warm sounding scale while the Minor scale is a little more evil sounding
I hope that helps because I haven't gone into any detail about the actual scales. To learn how to play these you can go here -> http://www.cyberfret.com/scales/basic/index.php
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